Comedian Russell Brand and singer Charlotte Church are among celebrities addressing thousands who joined an anti-austerity march in London on Saturday.
The march, predicted by organisers to be London's biggest in years, went from the Bank of England in the City of London to Westminster's Parliament Square, to be followed by a rally there.
A similar march was taking place in Glasgow.
Protesters were calling for the halting and reversal of spending cuts imposed by the previous Conservative-Liberal Democrat coalition and further measures proposed by Chancellor of the Exchequer George Osborne for the new Conservative government to enact.
Sam Fairbairn of protest organisers the People's Assembly said: "It will be the start of a campaign of protest, strikes, direct action and civil disobedience up and down the country.
"We will not rest until austerity is history, our services are back in public hands and the needs of the majority are put first."
Other set to address the rally include Len McCluskey, general secretary of Unite, firefighters' leader Matt Wrack, Labour leadership contender Jeremy Corbyn and Sinn Fein's Martin McGuinness.
Union leaders and campaign groups say they aimed to highlight the impact of previous cuts on public services, the NHS, welfare and education, and warn of the effects of the fresh cuts that Osborne is expected to announce in next month's Budget.
Green MP Caroline Lucas said: "This government is continuing to punish the poor for an economic crisis they didn't cause," according to an advance copy of her speech.
"It's time for all of us who oppose the government's self-defeating austerity programme to work together. That means more protests. It means taking action in our communities. And for those of us in parliament it means voting against further vicious cuts to our precious welfare state," Lucas said.
Marina Prentoulis represented Greek anti-austerity party Syriza on the protest.
"We have paid the price of the austerity policies in blood. The Greek government has the political and moral duty to put an end to the austerity measures that created a humanitarian crisis in Greece," Prentoulis said.
"The Greek government has the political and moral duty not to allow the further humiliation of the Greek people and to stand up and stand firm against any blackmail undermining democracy in Greece or anywhere else in Europe."